


Day 33: Dungeons, Dungeons, and More Dungeons

by PineTrain



Series: 40 Days of Pinecest [34]
Category: Gravity Falls
Genre: Alternate Universe - Reverse Falls, F/M, pinecest - Freeform
Language: English
Status: Completed
Published: 2017-04-06
Updated: 2017-04-06
Packaged: 2018-10-15 20:34:12
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,388
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/10557302
Author URL: https://archiveofourown.org/users/PineTrain/pseuds/PineTrain





	

It was an unfortunate turn of events, but Dipper and Mabel arrived in Gravity Falls with the knowledge that their Grunkles would not be there for the summer. There was technically still a chance the elder twins would be back from their work in the Bermuda Triangle before summer ended. For the moment, though, that was merely a distant hope. 

They knew it had to be extremely important since Stan and Ford would never leave for something menial so the younger pair didn’t hold it against the brothers. Still, they wanted to have some kind of interaction with their Grunkles, even if indirect. They would clean their rooms.

Stan had his in the annex to the Mystery Shack, while Ford slept in the refurbished bunker nearby. Despite their distance and the disparate personalities of their owners, both rooms shared the quality of being distinctly messy. Not dirty or gross, just messy, objects strewn about to the rhyme or reason of the occupant. Organizing them would not only clean things up a bit, it’d also give Dipper and Mabel a chance to maybe find something either brother had collected since the last time they had visited. An insight into their beloved family, perhaps even some blackmail… 

Dipper had Ford’s room. Knowing the eccentric scientist, he was sure he’d find something worth mentioning to Mabel later. He entered and, yep, it was a catastrophe of notes and half-finished inventions, a mysterious object here and there. Exactly what he expected. He cracked his fingers and began the arduous, but enjoyable, task. 

It was best to go top to bottom, Dipper figured. He would need to dust at some point after all. He cleared up a fair amount of space on the ground so he’d have a place to put stuff that might need rearranging. Looking around, he found a small step ladder and proceeded to examine the contents of the upper shelves. 

They were actually mostly empty besides a layer of dust. Ford was always either pacing the room or at his desk when he worked, depending on if he was thinking or calculating. Either way, he only put things he had very little use for on these shelves and there weren’t a lot of things he held onto that were of little use. Among them, though, was a familiar box. 

Dipper nudged it over towards him and cautiously regarded the box holding the Infinity-Sided Die, well aware of its power. In a way, this might be a safe place for it. There was no chance of any sudden movement knocking it off a table and it was quite obvious on such an barren shelf. It wouldn’t be hard to find. Still, Dipper decided to move it to another location, the risk of the box falling and breaking open far too great. It might land on 4 “surprisingly often” according to Ford, but Dipper had already seen one roll that caused a lot of trouble. 

It was too bad that Murphy’s Law decided to make an appearance the second someone attempted to be sensible. As he moved a foot down to the next step, the toe end of Dipper’s shoe caught the one he was currently on. His reflexes violently shot the foot to the lower step for stability, but this only exacerbated the problem. The drastic and unexpected shift in stance threw him off balance and his other foot flung into the air as he wobbled in a desperate attempt to prevent the impending fall. 

When it finally happened despite his efforts, the box launched from his grasp to bounce along the ground. The die flew from it in a spiral as it popped open from the force of the landing. Landing on the carpet, the die rolled a few feet as he recovered enough to turn onto his stomach and see what symbol was on top. The silhouette of a woman with a colorful pentagram on her chest. 

There was a blinding flash and when Dipper’s vision returned he saw a bewildered young woman looking around in a panic. “What in the hell!?!?!” she said.

It was Mabel. Or, she looked like Mabel, but different. Mabel was often theatrical, but this girl’s blue coat and black tights had an air of planning that didn’t fit Mabel’s style. Mabel was wild and unpredictable while somehow still cohesive and stunning. This outfit was striking, but in a more intentional way. As if to tantalize and distract instead of a simple expression of unbridled zeal for fashion. Beautiful, yet cold. 

And the tone of her voice. Less of confusion than it was of irritation, a weird confidence seeping through as if no matter the situation, she could handle it. The arrogance was only matched by the disgusted look she cast about the room. Mabel never talked like that. Neither did she have such a perfectly maintained face, minor blemishes dotting her from the various accidents she stumbled into. This girl looked like she’d never had so much as a light sunburn. 

Dipper shivered a bit as the doppelganger’s eyes fell upon him, his discomfort palpable. She opened her mouth as if to say something, but closed it as she quirked an eyebrow. Stepping over to him, she knelt down and used her black gloved hand to raise his hair over his forehead. 

Dipper flushed a little at the intense gaze she was giving him. No one had ever looked at him quite like this, especially not so close. He was relieved when she removed her hand and looked around the room. Relief that quickly left when he saw her fixate on the shifting die on the floor. 

Mabel carefully retrieved it and set it back into the box gently. Closing it with equal care, to Dipper’s astonishment an errant sock rose up behind her, a blue glow surrounding it just as the same glow emitted from Mabel’s hairband. The sock began unraveling into a series of strings that wrapped around the box, securing it tightly shut. Seemingly satisfied with the result, Mabel set the box down on a nearby table. 

“So,” Mabel said, “It seems you had an accident, Dipper.” 

“Mabes? You’re, uh, not Mabes… but… are?” Dipper said as he stood up. He thought he understood what was going on, but wasn’t sure. Had he pulled a Mabel from another dimension? 

Mabel smiled in a way that bothered Dipper, “I see you’re not too stupid, even if you’re clumsy. Yes, I’m not ‘your’ Mabel. I would wager I’m not too different from her since you aren’t too different from ‘my’ Dipper.” She gave him an up-down and grimaced, “I can only hope she wasn’t slapped with a shovel before she dressed herself as you apparently were.” 

Dipper decided he didn’t like this Mabel in the least. It was like talking to Pacifica when she was in bitch mode times 100. It felt so wrong to see this persona in his sweet and kind sister. There was also this weird gleam in her eyes hiding behind her disdainful glare. 

“There is something to be said for your sloppiness, though…” she mused, approaching him so smoothly he hardly noticed until she was uncomfortably close. “My Dipper is always so fastidious. So serious. He wouldn’t be caught dead looking like you. Certainly not by me.” Her fingers daintily settled on his chest and drifted down, Dipper too stunned by the sensation of them along his muscles to stop her. “Hmm, stronger than him too, it seems. No magic to assist you, after all. It is always interesting to try new things…” 

Okay, Dipper was definitely not thrilled about where this was going. Or he hoped he wasn’t as this mysterious, yet familiar girl slipped her arms under his vest and pulled him into a hug, her eyes boring into his. He realized what the gleam in her eyes was: Lust. 

“You interrupted us, you know,” Mabel said, humming as she ran her hands along his back. 

“Interrupted?” Dipper asked nervously. He squeezed his arms between them to set his hands on her shoulders. He didn’t have the wherewithal to push her away, but his arms gave him a bit of space in this most awkward of sibling hugs. 

Mabel smirked at his action. “Oh yes, brother dear,” she said coyly, “and for me to get back to my dimension, I know we have to fulfill some kind of condition.” She leaned in against his meagre resistance and whispered in his ear with icy breath, “I’m pretty sure it’s the fun kind.” 

Dipper stiffened and pushed her away. His face felt hot and his heartbeat was picking up pace. Mabel stumbled for a second, her face flashing through shock, then anger, before settling on smug amusement just as she balanced herself. 

“So, you don’t have that kind of relationship with your Mabel?” 

“Wh-what?” Dipper asked, though they both knew he had understood. 

“I guess it’s not terribly surprising. Incest and all,” Mabel said, closing her eyes and giving an understanding shrug. “But still,” she reopened her eyes with a seductive flutter, “how hard it must be for a growing young man to have such an attractive young woman so close by,” she flicked open the button of her jacket and spread it to reveal her undershirt, “What a shame that he can’t touch her how he wants to.” She reached in and opened two of the shirt’s buttons as she started walking towards him. 

Dipper backed away until he hit the wall, “I don’t know what you’re talking about. I’ve never thought about Mabel that way.” He was lying and she was totally right. But it was like she said! He was growing! This would pass once the hormone overload of puberty went away. 

“Oh. Really? Is. That. So?” 

Every word was punctuated by another button popping open and now Mabel stood directly in front of him with her shirt only loosely covering her. He tried to keep his eyes on her face and not on the gap revealing her skin. It wasn’t easy since she ran her fingers under the flaps of the garment when she brought her hands up from her waist. It wasn’t a lot, but the movement shifted the cloth enough to show off the curves of her breasts supported by a black, lacy bra. 

He stoically kept his eyes locked on hers, sweat beading on his forehead as he resisted the urge to even look using his peripheral vision. Her hands halted when they reached her collar, and with a sly grin she began to slowly pull her shirt open. 

“It’s okay, you know. My Dipper gets to see them all the time…”

Dipper could only hold on until she had moved the shirt over her shoulders and was letting it slip down her arms behind her back. His hand moved of its own volition to hover just in front of her chest. He still kept his eyes on hers, even as they mocked his quickly fading resistance. 

“Oh!” Mabel raised a hand to cover her mouth in false surprise. “You probably want to feel them directly, don’t you? How silly of me.” She reached behind her and undid the bra strap, the garment falling to the ground soon after. 

That was the final straw that broke Dipper’s will and he looked down to admire what he’d never even bothered to hope he could see. Mabel’s breasts were larger than he’d expected now that he saw them free. Dipper never understood cups and all that, so the best he could say was that they were just slightly too big to fit in his hand. It was a fact confirmed when she took his wrist and pulled his hand to her chest. 

His other soon followed and he gently ran them over her bosom, exploring her soft and pliant flesh. He noticed they were heavier than he expected too, experimentally lifting one. He kneaded them, marvelling at the feeling of them in his palms. His fingers slid to a nipple and he pinched it, Mabel’s moan bringing his eyes back up. 

She laughed, “How cute. You’re a virgin, aren’t you? I can’t even remember the last time my Dipper was so innocent about my body.” Mabel pulled his hands down to her waist as she stepped in with a predatory smile. She wrapped her arms behind his head and lifted her face so they were close enough that he felt that cold breath on his lips. “Why don’t we fix that little problem?” 

It was hard to understand how her lips could be so warm considering the chill he’d just felt. It was wonderful, though, and Dipper began pushing back against her. Her tongue pressed into his mouth and he squeezed her sides as he faltered when it wrapped his. It was all too soon that the kiss ended when they came up for air. 

He felt Mabel’s hand rubbing him through his shorts. “Hm, you seem like you’re ready to go, but let’s make sure,” she said, kneeling in front of him. He could only watch as she undid the button and zipper. She pulled his shorts and underwear down in one go and his cock leveled with her face, a small bead on the tip. 

She pulled off one of her gloves and he shuddered as she took hold of his length. She pumped a few times then squeezed lightly as she pulled slowly up from the base, the bead growing in size as she did so. Dipper whimpered when she lapped it off. She giggled and looked up at him, holding his member to her cheek. 

“I’ll let you off since it’s your first time, but you’ll want to use a better smelling soap down here when you finally get around to fucking your Mabel.” 

Dipper wasn’t sure how to take that comment, but it was hard to dwell on it as she dragged her tongue up his shaft and engulfed the head with her mouth. He hunched over slightly as she began bobbing back and forth, taking a little more in each time. He clenched at her shoulders, struggling to not use them for support as her wet tongue slid along him with each backward motion. He felt himself reach the back of her mouth and Mabel paused a second before slowly pushing even further. The tight muscles of her throat surrounded him and he groaned as a spasm of pleasure shot through his body. 

Suddenly, it was over. Mabel yanked back with a gasp, a trail of saliva leading from her mouth to his cock. She caught her breath and stood up. 

“Take off the rest of your clothes.” 

“Uh…” 

“That wasn’t a request,” she said harshly, pulling down her stockings and panties, a small trail of her own wetness clinging to them for a moment before snapping. 

Dipper followed her order. He was completely caught up in her momentum and his mind was fogging over with lust of his own. He felt her lips on his the very second his shirt passed over them. His arms were trapped in the cloth and he fumbled to throw it off so he could grasp her cheeks and push back with equal fury. 

He didn’t even think about doing it, but one of his hands had reached behind her to squeeze her ass. It was smaller than his Mabel’s, but firmer. Or at least that was his guess based on the times he’d chanced surreptitious glances at his sister’s rear when she wore a bikini. 

Mabel took his other hand and moved it between her legs. Dipper couldn’t believe how wet she was. His finger accidentally slipped into her with no effort at all. He studied her face as she closed her eyes and moaned while he explored her, another finger causing her them to pop open with an even louder moan. 

She fixed him with a greedy glare. “You suck at this. He’d have gotten me off at least once by now.” She mashed her lips against his for a few short seconds, then continued, “Probably for the best though. I don’t want this to end too soon. You don’t either.” 

Dipper was beginning to feel like nothing more than a tool for this girl’s pleasure. Probably the only reason he hadn’t realized that obvious truth already was because he was completely intoxicated by her blazing passion. Mabel pulled away to sit down in front of him. She lay on her back and gave a devious grin, her fingers spreading her pussy invitingly. Dipper fell to his knees between her legs, his cock throbbing in anticipation. He leaned over her, supporting himself with his arms near her shoulders. Looking down, he was mere inches from her entrance and could just barely feel the heat exuding from her.

“A c-condom. We should-” 

“I’ll worry about the goddamn birth control,” Mabel said irritably. She grabbed his dick with one hand and forcefully pulled him to her. Any reluctance he still had was moot when she wrapped her legs around his waist and pressed him into her. 

As soon as he felt the tight heat wrapping around him, Dipper instinctively shoved himself all the way in her. Mabel gasped and scratched his side with her sharp nails. He couldn’t believe how good it felt inside her. He trembled as he tried to regain control of his senses when she grabbed his face and kissed him again. 

“Feel good, virgin?” she said against his lips. 

Dipper was sick of her condescension and he pulled out to ram back into her, eliciting another gasp. He lifted himself up from her for better leverage and found a rhythm to his in and out motions. She was grinning at him with wild eyes and deeply flushed cheeks as he forcefully pounded into her. 

“I guess *ah!* you’re *nmph!* not so different *haaahhh* from him after all!” She rolled them over so she was mounted on top of him, “But you’re total shit at this, you fucking virgin!” 

She looked almost insane with lust. She alternated between moving up and down so he could fill her each time and furiously grinding atop him, the shifting connection making him twitch. It was all Dipper could do to try and keep pace with her, the clenching muscles around him making it hard to think about anything but pushing back into her. 

Eventually though, he reached his limit and grabbed her hips to pull her down as he thrust up to get as deep inside as possible when he came. Mabel moaned loudly as he shot his load into her, but when he was finally able to open his eyes he saw nothing but angry disappointment in hers. 

“I guess it is a bit much to expect a first timer to get me off,” she said with a sneer, squeezing her own nipple absentmindedly. 

Dipper struggled to think straight after such an overwhelming experience. The first sensible thought was a panicked one, “I-I came inside you!” 

“I told you to let me worry about that!” she snapped. She lifted herself off him and settled back on his legs. Reaching a hand down between her own, she brought up a small bit of leaking cum to examine. “I wonder how he’ll react to the possibility of ‘himself’ getting me pregnant instead of him…” she mused before grinning wickedly at Dipper. “I bet he’ll fuck me raw. For your sake, you better hope he never finds a way to this dimension.” 

A gasp came from the door and they both looked to see Dipper’s Mabel shocked with a hand over her mouth. Dipper had no idea what to do. How do you explain to your sister that you just had sex with an alternate version of her? 

Mabel was dragged into the room by that blue glow and the door slammed shut behind her. The doppelganger stood up and walked towards her. Mabel backed away just as Dipper had earlier, flinching at the touch of her mimic. The other girl merely laughed and pushed in further. 

“I’ve got to get home somehow,” she growled, gently running a finger down Mabel’s cheek as her other hand caressed his sister’s side, “and my Dipper is always telling me to go fuck myself…”


End file.
